456 



HISTORICAL GEOLOGY 



Back of the terminal moraines, over wide areas, the ground 

 moraine prevails, an undulating plain with gentle slopes. 

 Lakes and marshes strung on crooked, aimless streams are of 

 common occurrence (Fig. 474). Where the drift is thin, rock 

 hills may protrude, their rounded forms and polished, grooved 

 surfaces showing plainly the wear of the ice sheet upon them. 

 Elsewhere the entire surface is molded from the glacial 

 bowlder clay. In such districts there may be drumlins, 

 smooth, elliptical hills of till all trending parallel to the direc- 

 tion in which the ice was moving. 



The successive advances and retreats of the ice made the 

 distribution of these several features less simple than might be 



FIG. 474. Aimless drainage of 

 a glaciated region, eastern 

 Wisconsin. 



FIG. 475. Tree-shaped drain- 

 age systems in an unglaciated 

 region, northeastern Iowa. 



expected. Readvancing ice plowed over and defaced drumlins 

 and moraines which had been left at an earlier stage. Out- 

 wash deposits of stratified drift made in front of such an 

 advancing glacier were often worked over and buried under a 

 sheet of till. Later, outwash sands and gravels were spread 

 over moraines as the ice retreated. The escaping water ponded 

 behind terminal moraines cut channels in them here and there. 

 Some of the effects of glaciation on human affairs. Men- 

 tion has already been made of the excellent soils usually found 

 upon glacial lake floors and outwash plains. The finely 

 pulverized rock material left generally over the glaciated 



